1. Field Of The Invention
This invention directs itself to fan blade protection systems. In particular, this invention directs itself to a composite laminate structure bonded to a portion of a fan blade for substantially preventing erosion and subsequent structural damage. More in particular, this invention pertains to fan blade protection systems having an outer erosion resistant metallic layer bonded to an elastomeric energy dissipative layer. Further, this invention directs itself to composite laminate fan blade protection systems having a metallic outer layer of predetermined yield strength and predetermined thickness, to elastically deform responsive to particulate impingement, and be manually formable about the leading edge of the fan blade.
2. Prior Art
Fan blade protection systems are well-known in the art, as the problem of particulate erosion on fiber reinforced plastic blades is equally well-known. The most common particulates responsible for erosion damage to aerodynamic surfaces are water droplets. The erosion damage caused by water droplets is made up of two components: the first component is a depression formed by the direct impact of the droplet on the surface. The second component involves the deformation and flowing of the droplet itself as it moves out across the surface of the fan blade. These two components interact and result in a maximum pressure that occurs in a ring around the center of the initial impact point. The maximum pressure can be a factor of three times as great as the water hammering pressure. For non-metallic fan blades, the erosion takes the form of an initial surface depression with upraised edges. The edges can then be eroded away by the outflowing water from the droplet. Eventually, this action causes breaking of the underlying structural fibers which in turn leads to blade failure.
The best prior art known to the Applicants include U.S. Pat. Nos. 4,097,193; 4,667,906; 3,825,371; 4,318,672; 3,999,888; 3,689,178; 4,565,495; 4,342,542; 4,121,894; 4,671,471; 4,010,530; 3,859,005; 4,006,999; and, 2,767,436.
Some prior art such as that shown in U.S. Pat. Nos. 4,342,542; 4,318,672; 3,999,888; 4,006,999; 4,565,495; and, 3,825,371 are directed to fan blade protective systems which make use of metallic caps which may be secured releasably or fixably to the fan blade. The metallic caps protect the blade by absorbing the energy of impact and bending, the bending being resisted by the modulus of the steel, and the supporting substrate. The substrate can therefore still be damaged by the initial impact but is protected from the erosion effect of the flowing water.
Other prior art systems use elastomeric coatings which may be applied in the form of paints or pastes. Elastomeric coatings protect the blade by compressing and absorbing the energy of impact, but then the elastomeric surface is subject to erosion by the drop as it flows out across the surface. Typical materials used for these coatings are urethanes and neoprene rubber. However, these protecting coatings do not last very long, and are difficult to apply in the thicknesses required to be protective.
In other prior art systems such as U.S. Pat. No. 4,667,906, there are provided a replaceable tip for the leading edge of an aircraft comprising an abrasion shield fixably attached to a resilient cushion insert, spaced from the leading edge of the aircraft by a shim. The resilient cushion insert forms a damping arrangement for reducing damage to the aircraft leading edge when the replaceable leading edge tip is impacted. However, the metallic abrasion shield is not formed of a material having a high yield strength, since it is the design intent that the abrasion shield permanently deform on impact, and is therefore made replaceable. In addition, the resilient cushion insert is formed of a material having a very high spring rate, and therefore being relatively stiff deforms plastically rather than elastically. Further, this system does not comprise a composite laminate structure where both the abrasion shield and resilient insert cover a portion of the leading edge of the aerodynamic surface, as provided by the instant invention.